You can edit any old format and turn it into a icn file using a tool that comes free with the developer tools, which comes free with panther and tiger, and a seperate disk with jaguar.
On tiger, and probably panther, you can get it here as long as you have the developer tools.
Otherwise, it would be in a folder within the applications folder of the developer folder, Mac HD -> Developer -> Applications -> Utilities -> Icon Composer.

One thing to be careful of: last time I checked, Icon Composer didn't import .psd files correctly (the transparency didn't convert properly). I usually use TIFF format, single layered, with transparency.